1. Field of the Invention
The invention relates to a nonvolatile semiconductor memory device in which data is electrically rewritable and a method of manufacturing the same.
2. Description of the Related Art
An EEPROM (Electrically Erasable Programmable Read Only Memory) which writes and erases data electrically has been hitherto known as a nonvolatile semiconductor memory device. Further, a NAND-type flash memory which can be highly integrated has been known as one of the EEPROM. The memory cell of the NAND-type flash memory has a layer for the purpose of electrical charge storage and stores data by difference of threshold voltage according to the charge amount in the charge storage layer.
Recently, there is known the structure of providing memory cells stacked in a vertical direction to a substrate surface and providing a selection transistor above and below the memory cells (refer to Japanese Patent Application Laid-Open No. 2007-180389) in order to improve the recording density of a nonvolatile semiconductor memory device.
Generally, a plurality of memory cells are connected in series, sharing a source/drain diffusion layer, to form a NAND cell unit in the NAND-type flash memory. In the case of arranging the memory cells and the selection transistors in a vertical direction, the channel region of the selection transistors may be formed of polycrystal silicon or microcrystal silicon similarly to the memory cell. The selection transistor having the channel portion of the polycrystal silicon or the microcrystal silicon has an electrical property inferior to that having the channel portion of the single crystal silicon in a cut off characteristics, an on-current, an operation speed and so on. In the NAND-type flash memory of a stack structure, a good cut off characteristics is necessary in the selection transistors at the both ends (or one end) of the memory cells connected in series. Therefore, there is the case where the selection transistor having the channel portion of polycrystal silicon or microcrystal silicon cannot fill a necessary specification.